Datasheet

Chapter 3 Electrical Characteristics and Timing Specifications
MC9S08AC128 MCU Series Data Sheet, Rev. 4
Freescale Semiconductor 13
3.4 Thermal Characteristics
This section provides information about operating temperature range, power dissipation, and package thermal resistance. Power
dissipation on I/O pins is usually small compared to the power dissipation in on-chip logic and it is user-determined rather than
being controlled by the MCU design. In order to take P
I/O
into account in power calculations, determine the difference between
actual pin voltage and V
SS
or V
DD
and multiply by the pin current for each I/O pin. Except in cases of unusually high pin current
(heavy loads), the difference between pin voltage and V
SS
or V
DD
will be very small.
The average chip-junction temperature (T
J
) in C can be obtained from:
T
J
= T
A
+ (P
D
 
JA
) Eqn. 3-1
where:
T
A
= Ambient temperature, C

JA
= Package thermal resistance, junction-to-ambient, C/W
P
D
= P
int
P
I/O
P
int
= I
DD
 V
DD
, Watts — chip internal power
P
I/O
= Power dissipation on input and output pins — user determined
For most applications, P
I/O
 P
int
and can be neglected. An approximate relationship between P
D
and T
J
(if P
I/O
is neglected)
is:
P
D
= K  (T
J
+ 273C) Eqn. 3-2
Table 3-3. Thermal Characteristics
Rating Symbol Value Unit
Operating temperature range (packaged)
T
A
T
L
to T
H
–40 to 125
C
Maximum junction temperature T
J
150 C
Thermal resistance
1,2,3,4
80-pin LQFP
1s
2s2p
64-pin QFP
1s
2s2p
48-pin QFN
1s
2s2p
44-pin LQFP
1s
2s2p
1
Junction temperature is a function of die size, on-chip power dissipation, package thermal
resistance, mounting site (board) temperature, ambient temperature, air flow, power dissipation
of other components on the board, and board thermal resistance.
2
Junction to Ambient Natural Convection
3
1s - Single Layer Board, one signal layer
4
2s2p - Four Layer Board, 2 signal and 2 power layers

JA
61
47
57
43
81
28
73
56
C/W